1. Field of the Invention
The present invention generally relate to the field of fiber optic cables and, in particular, the present invention is directed to a fiber optic cable configuration having a central strength member with reduced radial stiffness.
2. Discussion of Related Art
Optical fibers are very small diameter glass strands that are capable of transmitting an optical signal over great distances, at high speeds, and with extremely low signal loss as compared to standard wire or cable networks. Optical fiber has found increasingly widespread application and currently constitutes the backbone of the worldwide telecommunication network. Because of this development, there has been a growing need for better quality optical fiber cables with a decrease in production time and costs, while ensuring adequate robustness for continued operation in increasingly harsh conditions. Proper signal transmission requires structurally sound jackets and protective covers to protect the optical fiber from potentially damaging external forces.
In general, optical fibers are manufactured from relatively large diameter glass preforms. Fiber optic preforms are generally made with concentric glass layers. The inner layer, or core, is made of a very high quality, high purity SiO2 glass. This high purity core is the portion of the optical fiber in which the optical data is transmitted. Concentrically positioned around the high purity core is a second layer of glass, or cladding, with a lower index of refraction than the inner core, and generally is less pure. The difference in refraction indices between the core and cladding allows the optical signals in the core to be continuously reflected back into the core as they travel along the fiber. The combination of the core and cladding layers is often referred to as the xe2x80x9cprimary preform.xe2x80x9d The optical fiber is formed by heating and softening a portion of the preform, and rapidly drawing the softened portion with specialized equipment. The length of the drawn optical fiber is typically several thousands of times the length of the primary preform. The aggregate of the optical fiber, jackets and additional integrated mechanical supports is typically referred to as an optical fiber cable. An integral part of the optical fiber cable configuration is a central strength member. The central strength member (CSM) is traditionally used to provide protection to the cable against strains arising from material contraction at low temperatures, as well as under cable bending and tension forces, which are often present during cable installation conditions.
A traditional optical fiber cable configuration 10 is shown in FIG. 1. An outer jacket 12 is provided to enclose and protect a plurality of buffer tubes 14. The buffer tubes may contain loose optical fibers, or optical fiber ribbons 16. The buffer tubes 14 are radially disposed around a central strength member (CSM) 18. The CSM is commonly made of glass reinforced plastic (GRP) and is used to provide strength and support to the cable configuration 10. Each of the buffer tubes 14 may contain loose fibers 16 or fiber ribbons. U.S. Pat. No. 5,621,841 discloses an optical fiber configuration having a CSM. The buffer tubes are stranded or wrapped around the CSM. The buffer tubes are enclosed by an armor layer and an outer sheath.
Recently, cable manufacturers have been attempting to increase the number of fibers per cable, in addition to reducing the amount of materials used, so as to limit the size of the cables. As a result, buffer tubes have been made to have thinner protective walls, which are commonly made of polypropylene and polybutylene terephthalate (PBT). It has recently been observed that cable configurations designed according to traditional standards are prone to suffer severe damage during installation and sheave testing. Specifically, it has been found that when the cable configuration is compressed in transverse or radial directions, for example, when bent around a sheave or subjected to an external crushing force, the buffer tubes become permanently flattened or indented. In many cases, the buffer tubes tear open, which allows for fiber bundles to protrude out from the buffer tubes, resulting in significant attenuation problems.
A main cause of the damage to the buffer tubes is due to a force exerted onto the buffer tubes by the CSM. As disclosed in U.S. Pat. No. 5,621,841, the CSM is often made to be incompressible in compression with other elements of the cable. In other words, the radial stiffness of the CSM is much higher than that of the buffer tube. As a result of the CSM being incompressible in comparison with other elements such as the buffer tubes, external compression forces are not absorbed by the CSM, but instead are transmitted from the CSM to the buffer tubes and absorbed by the buffer tubes. As can be seen with reference to FIG. 2, when a compression force F is applied to the cable configuration 10, certain buffer tubes 14 are crushed because the rigid CSM 18 transfers a compression force to the buffer tubes 14, which means that the buffer tubes 14 must absorb the force. Accordingly, the buffer tubes 14 often collapse and recess inwardly causing the optical fibers 16 to exert forces upon the inside of the buffer tubes 14. When either the force of the CSM upon the buffer tubes 14 or the internal force exerted by the optical fibers 16, is large enough, the buffer tubes 14 split open.
Thus, what is needed is a CSM that does not subject the buffer tubes to unacceptable radial forces, when the cable is bent or crushed.
The present invention is directed to eliminating the above problems associated with the high fiber count optical cables. Thus, the invention improves the quality of the optical fiber cable and provides for a cable configuration that can withstand forces, such as those induced by installation and sheave loads.
The present invention addresses the above problems by providing a modified strength member for an optical fiber cable. The modification is based on the substitution of the conventional solid rod configuration CSM with a hollow tube configuration CSM. The tube contains one or more GRP or other strength rods, positioned loosely in the tube. To avoid water penetration or a xe2x80x9cwater hosexe2x80x9d effect, the tube is filled with a gel or water-absorbing powder or any other water barrier material known in the art.
The present invention further provides for an optical fiber cable configuration having an outer jacket with at least one buffer tube disposed within the outer jacket. One or more optical fibers are positioned in the buffer tube. The buffer tube is stranded around a central strength member, which is disposed longitudinally along the axis of the outer jacket. The CSM has a hollow portion with at least one strength rod loosely positioned within the hollow portion. A gel or water-absorbing powder may be used as a water barrier in the CSM.
The present invention still further provides for an apparatus for an optical fiber configuration including an outer jacket, at least one buffer tube disposed inside of the outer jacket, at least one optical fiber positioned along the buffer tube, and means for strengthening. The means for strengthening is disposed longitudinally along the cable axis, and is operative to have a degree of deformation when a load is applied to the optical fiber configuration. The buffer tube is also operative to have a degree of deformation when the load is applied to the optical fiber cable. According to the present invention, the means for strengthening has a higher degree or the same degree of deformation as the buffer tube when the optical fiber cable is subjected to radial and transverse loads.
The present invention even further provides for a strength member for an optical fiber cable having a tube with a hollow portion and a wall portion, wherein a strength rod is positioned within the wall portion. Alternatively, the invention provides for a strength member for an optical fiber cable having a tube with a hollow portion and a wall, wherein a strength rod is embedded into an inner circumferential area of the wall so as to partially extend into the hollow portion.
These and other objects and advantages of the present invention will become apparent in the course of the following description.